Hydrogel contact lens materials prepared with N-vinyl-2-pyrrolidone (NVP) are expected to have a relatively high water content, and thus, an acceptable level of oxygen permeability. For example, NVP is often copolymerized with an alkyl acrylate or methacrylate such as methyl methacrylate to provide lens materials that typically have a water content of 50% to 80% by weight. However, such copolymers are difficult to synthesize in a homogeneous controlled manner because of the difference in the polymerization reaction rates between the N-vinyl groups of NVP and the acryloyl or methacryloyl groups of the alkyl acrylate or methacrylate. One typically observes a phase separation and a corresponding decrease in the transparency of the polymeric lens material, or the mechanical properties of the lens material deteriorates as the lens absorbs water.
In an attempt to overcome the differences between NVP and the alkyl acrylates/methacrylates, U.S. Pat. No. 4,547,543 describes the use of N-methyl-3-methylene-2-pyrrolidone (NMMP). It is stated that NMMP has a polymerization reaction rate more in-line with acrylate/methacrylate monomers, and still provides the necessary hydrophilic character desired of NVP. Accordingly, the '543 patent describes a copolymer consisting essentially of: (a) from 50 to 95 parts by weight of the total monomer units, of which, 25 to 100 parts is NMMP and 0 to 75 parts NVP; and (b) from about 5 to about 50 parts, by weight of the total monomer units, of reinforcing monomer units consisting essentially of at least one monomer selected from the group consisting of alkyl acrylates, alkyl methacrylates, styrene, alkyl styrenes, benzyl acrylate and benzyl methacrylate.
U.S. Pat. No. 3,949,021 describes a somewhat different approach to the above mentioned problem. The '021 patent describes trapping or encapsulating an already formed, water insoluble polymer (e.g., poly(methyl methacrylate), polystyrene, or poly(vinyl acetate) in poly(NVP). Also, U.S. patents by McCabe et al. (U.S. Pat. No. 6,822,016 and U.S. Pat. No. 7,052,131) describe a process of making a polymeric, ophthalmic lens material from a high molecular weight hydrophilic polymer and a silicone monomer. The McCabe process polymerizes the silicon monomer in the presence of an already formed hydrophilic polymer, e.g., poly(NVP) having a molecular weight of no less than about 100,000 Daltons.
Conventional polymer formulations that include two or more free-radical, monomers with two very different reactivity ratios and a single crosslinking agent can provide a polymer in which the two monomers coexist as essentially two homopolymers. During the initial stages of the polymerization reaction one monomer preferentially reacts with the crosslink agent, and only after that monomer is nearly consumed does the second monomer begin to react with the crosslink agent. In some instances, the large difference in the reactivity ratios of the crosslink agent and the second monomer can provide a polymer with relatively large amounts of unreacted second monomer or oligomer, which then must be extracted from the polymer. This is very inefficient in terms of production cost (yields) and can be detrimental to the material properties. Also, it is often difficult to prepare a polymer from one production batch to another and stay within design specifications under such variable reaction conditions.
The theoretical composition of the polymer formed by the reaction of two different monomers (one of which is a crosslink agent) is determined by the following:dn1/dn2=(N1/N2)(r1N1+N2)/(r2N2+N1)where n1 is the moles of monomer 1 in the copolymer, n2 is the moles of monomer 2 in the copolymer, N1 and N2 are the number of moles of monomers 1 and 2, respectively, in the monomer mixture, and r1 and r2 are the monomer reactivity ratios. The reactivity ratios are defined in terms of propagation rate constants, k11, k12, k22 and k21, according to the following polymerization reactions.M1*+M1→M1*  k11 M1*+M2→M1M2*  k12 M2*+M2→M1M2*  k22 M2*+M1→M2M1*  K21.and r1=k11/k12 and r2=k22/k21.
The invention overcomes the shortcomings that result from attempts to copolymerize at least two monomers with a single crosslink agent if one of the two monomers has a very different reactivity ratio relative to the crosslink agent.